In many stationary and mobile devices, the radio transmitter and the radio receiver may be active at the same time due to the various radio frequency (RF) systems that are present in the device. As the number of RF systems has increased, not only is it likely that simultaneous receiving and transmitting of different signals is present but also that the frequency difference between them is insufficient for simple isolation to be achieved using inexpensive duplexers and filters, if at all possible. A well know technique to reduce the interference and to improve the coexistence of different RF signals is to use a pair of transmission (TX) antennas and a single reception (RX) antenna as illustrated in FIG. 1 that shows a prior art communication system 10.
The signal to be transmitted (transmitted signal 31) is split (by splitter 14) and applied to two TX antennas A and B (12 and 13) which are spaced greater than half a wavelength apart. TX antennas A and B 12 and 13 transmit RF radiation—22 and 23 respectively. The RX antenna 11 receives a wanted (far field) signal 21 as well as unwanted receive signals A and B 23 and 25 from TX antennas A and B.
In FIG. 1, the distance between the TX antenna B 13 and the RX antenna 11 is D 41 and the distance between the TX antenna A 12 and the RX antenna 11 is D+λ/2 42, where λ is the wavelength of the signal. Hence at the RX antenna 11, the signal (unwanted receive signal A 23) received from TX antenna A 11 is exactly out of phase with the signal (unwanted receive signal B 25) received from TX antenna B 13. The result being that the sum of the two signals 23 and 25 received from TX antennas A and B is at a much attenuated level compared to the signal at either antenna A or B. Attenuations of about 35 dB can be obtained when using this scheme. FIG. 4 illustrates a summation of leakage signals by RX antenna 11.
Referring back to FIG. 1, also of concern is that the closer a TX antenna is to the RX antenna, the higher the unwanted signal that has to be attenuated and the larger the difference between the signals at the two receive antenna.
In general, referring to FIG. 1, the signal strength at the RX antenna 11 from TX antenna B 13 will be stronger than that at RX antenna 11 due to the signal from TX antenna A 12. This would have to be compensated for by attenuating the power of the TX transmission from TX antenna B 12, as shown by the variable attenuator 15 that attenuates signals 33 to provide attenuated signal 34 while signal 32 that is fed to TX antenna A 12 is not attenuated. Variable attenuator 15 is located in the signal path from the splitter 14 to the base of TX antenna B 13. This could be seen as inefficient and a waste of energy. For example, if the distance D is λ/4, then the distance of TX antenna B from the RX antenna is 3λ/4, or 3 times the distance. This would result in the signal 23 at the RX antenna from TX antenna A being in the order of 9 dB higher than the signal 25 from TX antenna B 13. If the relative distances were 2λ and 5λ/2, then the difference is less than 2 dB, but the antenna spacing has increased.
Therefore in order to achieve deep attenuation, the TX signal 34 applied to TX antenna B would need to be attenuated such that the signals received at the RX antenna from TX antennas A and B are equal in amplitude.
The example given shows TX antennas 12 and 13 in line, it is possible to have the antennas located at the corners of a triangle, as illustrated in FIG. 2.
The distance from TX antenna A to the RX antenna must be an odd multiple of λ/2 compared to the distance from TX antenna B to the RX antenna. In FIG. 2 the distance between RX antenna A and TX antenna B 13 is D and the distance between RX antenna A and TX antenna B 13 is {(2N+1)*λ/2}+D, N can take any integer value as well as a value of zero. The separation of the two TX antennas must be sufficient such that the signals are not correlated in the far field. The separation distance so as to assume the signals will be uncorrelated is usually assumed to be λ/2 (reference “Foundations of Mobile Radio Engineering”, Michael Yacoub, 1993 CRC Press, page 179). It is also possible to have the TX antennas and the RX antenna in opposite polarities. The RX antenna could be positioned for horizontal polarization and the two TX antennas could be positioned for vertical polarization, for example. This reduces the unwanted coupling between the antennas.
It is also possible to use two TX antennas and one RX antenna with the same equidistance spacing, as shown in FIG. 3 that shows a prior art communication system 10′.
The two TX antennas, A and B 12 and 13, are equidistant, D, from the RX antenna 11. The signal to be transmitted 31 is split into two equal components. The signal applied to TX antenna A is first subjected to a phase change 16 of +π/2 radians and the signals applied to TX antenna B is first subjected to a phase change 17 of −π/2 radians. The net result is that the two signals will be received at the RX antenna 11 at equal amplitude but at a phase difference of it radians, i.e. ideally out of phase, and hence will cancel.
Other cancellation schemes have been proposed. For example, Osama N. Alrabadi et al, (“Breaking the Transmitter-Receiver Isolation Barrier in Mobile Handsets with Spatial Duplexing”, Smart Antenna FrontEnd (SAFE) project within the Danish National Advanced Technology Foundation—High Technology Platform), describe a scheme where the TX is equipped with extra redundant antennas and by properly weighting the TX antennas the TX signal is selectively attenuated in the RX direction. Similar to the more simple cancellation scheme described above, this scheme relies on multiple TX antennas.